Photochromism is a reversible phenomenon of a compound which quickly changes its color when it is irradiated with light containing ultraviolet rays, such as sunlight or light of a mercury lamp and resumes its initial color when it is placed in a dark place being no longer irradiated with light. A compound having this property is called photochromic compound and is used as a material of photochromic plastic lenses.
The following properties are required for the photochromic compound used for the above application.
(1) Having a low visible coloring degree (initial color) when not irradiated with ultraviolet rays.
(2) Having a high coloring degree (color density) when irradiated with ultraviolet rays.
(3) Having a high color-developing rate which defined as an increasing rate of the color density during the period from directly after the irradiation of ultraviolet rays to the saturation of the color density.
(4) Having a high fading rate which defined as a returning rate of the color density after stopping the irradiation of ultraviolet rays.
(5) Having a good repeating durability of the reversible phenomenon.
(6) Having a good dispersibility into a monomer that is used (i.e, having an ability to dissolve into a monomer composition which is the host material after cured at a high concentration).
As for the photochromic plastic lenses, further, neutral tints such as brown and amber are preferred as color tones in the state of developing color. Naturally, therefore, what color should be developed is a very important factor for the photochromic compound.
When a color tone is adjusted by mixing a plurality of photochromic compounds together, a change in the color tone (color deviation) at the time of fading stemming from different properties of the photochromic compounds and a change in the developing color tone after aging due to a difference in the durability of the compounds are occurred. To solve the above problem, a photochromic compound which by itself develops a neutral tint is important.
The photochromic compound which by itself develops a neutral tint can be exemplified by a chromene compound of the following formula (A) (see patent document 1) and a chromene compound of the following formula (B) (see patent document 2).

There have been obtained photochromic plastic lenses having favorable photochromic properties by curing and molding (cast-polymerizing) a curable composition obtained by dissolving the photochromic compounds in a radically polymerizable monomer relying upon the thermal radical polymerization (see patent documents 1 and 2).
The method (also called in-mass method) of producing the photochromic plastic lenses by the cast polymerization employed by the above patent documents is one of the representative methods of producing the photochromic plastic lenses imposing, however, a limitation on the polymerizable monomers that can be used for obtaining favorable photochromic properties. In recent years, a coating method free of the above limitation is drawing attention as a method of producing photochromic plastic lenses (see patent document 3). According to the coating method, a coating agent comprising a curable composition containing a photochromic compound is applied onto the surface of the lens, and the coating is cured to form a photochromic coating to thereby impart photochromic property to the lens substrate. In principle, therefore, there is no limitation on the lens substrate provided the coating can be closely adhered thereto.    Patent document 1: pamphlet of International Laid-Open WO00/15628    Patent document 2: U.S. Pat. No. 7,048,876    Patent document 3: pamphlet of International Laid-Open WO2003/011967